The ability to generate and manipulate light with electricity by Sir Humphrey Davy 200 years ago sparked a century of development by the likes of Thomas Edison, Joseph Swan, Sandor Just (tungsten filaments), and Irving Langmuir (inert gas instead of vacuum) leading to the establishment 100 years ago of tungsten filament lamps, which as the dominant light source have fundamentally shifted how people live, work, play. However, the efficiency of such light sources is woefully low. For example a 60 W incandescent light is only 2.1% efficient, a quartz halogen only 3.5%, in terms of generating light within the visible spectrum of the human eye. Accordingly today there is a massive worldwide campaign to have incandescent lights replaced wherever possible by compact fluorescent lights (CFL) which have an efficiency of approximately 22% thereby reducing energy consumption significantly.
However, whilst CFLs provide an immediate and visible statement by Governments and other organizations worldwide that they are addressing global climate change, environmental issues etc they are not a panacea. Amongst the disadvantages of fluorescent lights are frequent switching limiting lifetime, health and safety from the mercury content, UV emissions which affect some materials, flicker affecting individuals with autism, epilepsy, lupus, chronic fatigue syndrome, and vertigo, radio interference, operating temperature where efficiency drops with increasing/decreasing temperature from room temperature, non-operation at below freezing, low-luminance requiring long tubes and limiting power output, dimming, and recycling through the phosphor and mercury.
However, a monochromatic solid state light source within the visible wavelength range can achieve in principle an efficiency approaching 100%. Additional to energy consumption such solid state light sources should also reduce consumption of precious metals, reduce recycling as well as address health and safety issues. Beneficially solid state light sources by virtue of their small size, low weight, and low voltage operation can also be employed in a wide range of situations where incandescent or CFL lights cannot. At present niche applications such as holiday decorations in conjunction with indicator lighting in panels, back lighting in LCD displays etc mean that solid state lighting sales today account for only approximately 2% of the current lighting market and will grow to only approximately 3% in 2011.
Despite this solid state lighting is a massive market which according to NextGen Research (“LEDs and Laser Diodes: Solid State Lighting Applications, Technologies, and Market Opportunities”, February 2009, http://www.nextgenresearch.com/research/1001995-LEDs_and_Laser_Diodes) forecasts the overall solid-state lighting (SSL) market will achieve worldwide revenues topping $33 billion by 2013. The illumination segment of the LED market will see compound annual growth rate (CAGR) of nearly 22% in the 2009-2013 timeframe as cities worldwide shift their streetlights to these more energy-efficient and ecologically friendly solutions. However, the majority of this growth will be generated from niche lighting applications including architectural, task lighting, medical and off-grid lighting applications rather than the residential lighting market according to The Strategy Analytics (“LED Device and Material Market Trends”, June 2009, http://www.strategyanalytics.com/default.aspx?mod=ReportAbstract Viewer&a0=4788).
As such the majority of the lighting market remains inaccessible despite the considerable research effort and investment have been expended. This arises due to the challenges in realizing suitable LED technologies and devices using conventional quantum well structures, these including the relatively low internal quantum efficiency of these structures, the low light extraction efficiency realized, and relatively high device fabrication costs. Additionally to achieve a “white” LED today the devices will generally employ a phosphor-conversion scheme, which sets the ultimate quantum efficiency of white LEDs to below 65%. In this regard, the high luminescence efficiencies, low fabrication costs, and processability of semiconductor nanostructures, including quantum dots and nanowires, have made them promising candidates for future lighting devices and the subject of considerable research and development.
With the recent discovery that the band gap of indium nitride (InN) at approximately 0.7-0.8 eV, see for example J. Yu et al in “Unusual Properties of the Fundamental Band Gap of InN,” (Appl. Phys. Lett., Vol. 80, p. 4741, 2002) and T. Matsuoka in “Optical Bandgap Energy of Wurtzite InN” (Appl. Phys. Lett., Vol. 81, p. 1246, 2002), the epitaxial growth and characterization of InN nanowires and whiskers has become the focus of significant research, including for example T. Stoica et al in “MBE Growth Optimization of InN Nanowires” (J. Crystal Growth, Vol. 290, p. 241, 2006), R. Calarco et al in “GaN and InN Nanowires Grown by MBE: A Comparison” (Appl. Phys. A, Vol. 87, p. 499, 2007), C-Y Chang et al in “Electrical Transport Properties of Single GaN and InN Nanowires” (J. Elect. Materials, Vol. 35, No. 4, p. 738, 2006), F. Werner et al “Electrical Conductivity of InN Nanowires and the Influence of the Native Indium Oxide Formed at Their Surface” (Nano. Lett., Vol. 9, p. 1567, 2009) and J. Grandal et al “Accommodation Mechanism of InN Nanocolumns Grown on Si (111) Substrates by Molecular Beam Epitaxy” (Appl. Phys. Lett., Vol. 91, 021902, 2007).
When compared to other nitrides with group IIIA elements including aluminum, gallium and boron, InN exhibits the highest electron mobility (4400 cm2V−1 s−1 at 300 K), the smallest effective mass, and the highest saturation velocity. These properties make InN an excellent candidate for next generation of nanophotonic and nanoelectronic devices, including chip level nanoscale lasers and high-speed field effect transistors. Additionally, the band gap of InN at approximately 0.7 eV (1750 nm) when compared with GaN at approximately 3.3 eV (370 nm) means that the ternary alloy InGaN can be continuously tuned from approximately 0.7 to 3.3 eV, matching almost perfectly to the solar spectrum. Therefore, InGaN has also emerged as a promising material for future high-efficiency full solar spectrum solar cells, E. Trybus et al “InN: A Material with Photovoltaic Promise and Challenges” (J. Crystal Growth, Vol. 288, p. 218, 2006) as well as for broadband light sources (e.g. white LEDs or UV/visible and visible/IR LEDs).
It should be apparent to one skilled in the art that in order to provide broadband spectrum photonic devices that it should be structured so that the material at the front of the device emits/absorbs the shortest wavelengths and progressively longer wavelengths are emitted/absorbed by layers within the solar cell towards the lower most surface. As such, the material within a broadband device may grade from InxGa1-xN where x≈1 to InyGa1-yN where y≈0, or a limited range therein. As such it is necessary to grow InxGa1-xN nanowires onto the substrate of the photonic device.
Within the prior art InxGa1-xN nanowires have been predominantly grown using the conventional approach of a foreign metal catalyst via the vapor-liquid-solid growth mechanism, see for example J. Li et al in U.S. Pat. No. 6,831,017 entitled “Catalyst Patterning for Nanowire Devices”, L. Romano et al in U.S. Pat. No. 7,344,961 entitled “Methods for Nanowire Growth”, C. Liang et al in “Selective-Area Growth of Indium Nitride Nanowires on Gold-Patterned Si(100) Substrates” (Appl. Phys. Lett., Vol. 81, p. 22, (2002) and X. Cai et al in “Straight and Helical InGaN Core-shell Nanowires with a High In Core Content” (Nanotechnology, Vol. 17, p. 2330, 2006). They have also been formed spontaneously under nitrogen rich conditions; see for example C-K Chao et al “Catalyst Free Growth of Indium Nitride Nanorods by Chemical Beam Epitaxy” (Appl. Phys. Lett., Vol. 88, p. 233111, 2006) and S. Hersee et al in U.S. Pat. No. 7,521,274 entitled “Pulsed Growth of Catalyst-Free Growth of GaN Nanowires and Application in Group IIIA Nitride Semiconductor Bulk Material.”
Whilst the influence of growth parameters on the structural and optical properties of InxGa1-xN nanowires has also been extensively studied, epitaxial InN nanowires grown according to the prior art exhibit tapered morphology, with a large variation in the wire diameter along the wire length, see for example T. Stoica et al in “MBE Growth Optimization of InN Nanowires” (J. Crystal Growth, Vol. 290, p. 241, 2006) and J. Grandal et al “Accommodation Mechanism of InN Nanocolumns Grown on Si (111) Substrates by Molecular Beam Epitaxy” (Appl. Phys. Lett., Vol. 91, p. 021902, 2007), and demonstrated spectral linewidths for these InN nanowires are commonly in the range of 60-100 meV. The extremely large inhomogeneous broadening observed makes it difficult to study the fundamental properties of InN, including the temperature dependence of the band gap and the electron concentration. Additionally, the poorly defined wire geometry leads to uncontrolled electrical and optical properties, severely limiting their device applications.
Further the direct growth of InxGa1-xN on silicon, the most suitable substrate for InxGa1-xN in terms of lattice and thermal mismatches, has been further complicated by the development of an amorphous SiNx layer during the initial stage of growth, see J. Grandal et al. Attempts to adjust these growth techniques, either by the intentional introduction of hydrogen or buffer layers, such as GaN or AlN, have met with limited success.
The prior art whilst demonstrating InxGa1-xN nanowires can be grown has not yet demonstrated them with high quality and constant cross-section nor have they been grown on suitable substrates for low cost semiconductor processing. These developments to date being hindered to a large extent by the relatively low decomposition temperature of InN (approximately 500° C.-550° C.) as well as the very high migration rate of indium. Additionally, the prior art does not provide an effective means to control the growth and properties of InN nanowires spontaneously formed under nitrogen rich conditions.
Accordingly it would be beneficial to provide a means of growing high quality, uniform InxGa1-xN nanowires directly upon silicon substrates without the requirement for predeposition of a catalyst. It would be of further benefit for the growth rate and properties of the InxGa1-xN nanowires to be controlled through the parameters of the growth process and for the growth to continue despite the growth of the SiNx layer.
It would be of further benefit for the growth rate and properties of InN and InGaN nanowires to be controlled through the parameters of the growth process and for the growth to continue despite the growth of the SiNx layer. As discussed supra graded InGaN nanowire structures would allow full solar spectrum solar cells to be implemented with a single growth process thereby greatly increasing the efficiency of solar cells and reducing their costs.
As discussed supra graded InGaN nanowire structures would allow broad spectrum photonic devices to be implemented with a single growth process thereby greatly increasing the efficiency of solar cells, white LEDs etc and reducing their costs. In solid state lighting applications the ultimate goal is a high efficiency white LED, typically operating from approximately 400 nm to approximately 750 nm. However, according to the prior art simple LED structures whilst offering a fairly broad wavelength range operate at relatively low efficiencies and typically employ three LED devices are required to even cover a substantial portion of the wavelength range to which the human eye responds, the so-called visible wavelength range, which is 380 nm to 750 nm. As such red, green and blue centered LED devices are typically used to create the impression of white. Blue LEDs were the last to be developed based upon InGaN structures. These blue LEDs also form the basis of phosphor based white LEDs. However, increasing the efficiency of LEDs by the introduction of quantum confined structures, such as quantum wells, multi-quantum wells etc also results in a narrowing of the optical emission from the source thereby requiring that number of sources required to “blend” together for the desired white light increases, along with the cost and complexity of the devices overall.
It is within this context that semiconductor quantum dots, nanometer sized semiconductor particles which act as a very small “box” for electrons, and potentially the most efficient light sources ever developed have formed the subject of significant research. A specific class of quantum dot is the colloidal quantum dot created by solution phase chemistry. Much of the appeal of the colloidal quantum dot is that it can be readily integrated with other technology platforms at very low cost and that by varying the physical dimensions of the quantum dots they can be made to emit at points across the entire visible spectrum. Accordingly providing colloidal quantum dots with a range of dimensions within the same localized region acts to provide the required multiple sources to “blend” together to provide the illusion of a white light source. Recent work by R. R. Cooney et al entitled “Gain Control in Semiconductor Quantum Dots via State-Resolved Optical Pumping” (Phys. Rev. Lett., Vol. 102, 127404, 2009) has shown that quantum dots are in fact the most efficient material for generating gain ever measured. The use of quantum dots as white light emitters and in LEDs has also been proposed and demonstrated, see for example S. Sapra et al (Adv. Mater., Vol. 19, p. 569, 2007), M. J. Bowers et al (J. Am. Chem. Soc., Vol. 127, p. 15378, 2005), S. Coe et al (Nature, Vol. 420, p. 800, 2002), N. Tessler et al (Science, Vol. 295, p. 1506, 2002), and M. C. Schlamp et al (J. Appl. Phys., Vol. 82, p. 5837, 1997).
However, whilst colloidal quantum dots are themselves efficient an optical emitter employing them can only be efficient if the colloidal quantum dots are optically pumped with an efficient emitter at the appropriate wavelength. As noted supra InGaN nanowire structures can form the basis for very high efficiency emitters that cover the wavelength range from near UV (370 nm) to the near infra-red (1750 nm).
Accordingly it would be beneficial to provide a combination of colloidal quantum dots with high quality, uniform InGaN nanowires that can be grown directly upon silicon substrates without the requirement for predeposition of a catalyst. Such a combination beneficially combines high efficiency InGaN based nanowire LEDs for pumping highly efficient colloidal quantum dot emitters which when formed from multiple dimensions yield emission across the visible spectrum, thereby providing a high efficiency “white” LED.
To date significant progress has been made in demonstrating blue and blue-green LEDs using InGaN to create the third LED within a red-green-blue combination (RGB) approach in forming a white LED. LEDs for the remainder of the RGB combination being manufactured typically from AlGaInP and GaAsP based quaternary semiconductor systems for yellow-orange LEDs and GaAsP and AlGaAs for red LEDs. However, as noted supra InGaN allows bandgap tuning across the visible spectrum and into the UV/infrared. Within the prior art InGaN/GaN LEDs have exhibited very low internal quantum efficiencies in the green, yellow and red wavelength ranges, see for example P. T. Barletta et al (Appl. Phys. Lett., Vol. 90, p. 151109, 2007) and C. Wetzel et al (MRS Internet J. Nitride Semicond. Res., Vol. 10, p. 2, 2005). One of the primary causes for this low efficiency is the strain-induced polarization field in InGaN/GaN quantum wells and the resulting quantum confined Stark effect, which leads to a spatial charge separation.
In this regard, InGaN/GaN quantum dot heterostructures have drawn considerable attention; see for example K. Tachibana et al (IEEE J. Sel. Top. Quantum., Vol. 6, p. 475, 2000), N. Grandjean et al (Proc. IEEE, Vol. 95, p. 1853, 2007) and Q. Wang et al (Appl. Phys. Lett., Vol. 93, p. 081915, 2008). This is based upon their providing strong carrier confinement and being identified, although with some debate, as the emission mechanism for the high efficiency InGaN blue and blue-green LEDs and lasers. However, to date three-dimensional InGaN/GaN quantum dot heterostructures obtained by self-organization using Stranski-Krastanow growth or phase segregation induced In-rich clusters have yielded similar results to bulk planar InGaN/GaN quantum well heterostructures. As such high efficiency long wavelength (>550 nm) emission has been severely limited by the presence of large densities of misfit-dislocations related to the large lattice mismatch (approximately 11%) between InN and GaN. However, significantly reduced defect densities can be achieved in InGaN nanowire heterostructures, due to the effective lateral strain relaxation, see Y. Chang et al in “Molecular Beam Epitaxial Growth and Characterization of Non-Tapered InN Nanowires on Si(111)” (Nanotechnology, Vol. 20, p. 345203, 2009). This reduced strain distribution also leads to a weaker piezoelectric polarization field.
As noted supra InGaN nanowires offer advantages for LED manufacturing, including high light extraction efficiency and the compatibility with low cost, large area Si substrates, see Y. Chang et al (Appl. Phys. Lett., Vol. 96, p. 013106, 2010), and can form the basis of either discrete LEDs or LED based white LEDs in combination with colloidal quantum dots. However, to date such nanowire structures have reported extremely low internal quantum efficiency (<10%), due to the lack of effective carrier confinement in the wire axial direction as well as the nonradiative recombination associated with the presence of surface states. However, with the ability to form nearly defect-free InGaN nanowires as discussed supra then it is proposed that InGaN quantum dots directly embedded in InGaN nanowires would provide a route to realizing high efficiency green and red emission sources. Within the prior art no such nanoscale heterostructures have been reported.
According to embodiments of the invention based upon the ability to form defect-free InGaN nanowires directly onto silicon substrates without foreign metal catalysts it would be beneficial to modify the growth process such that nearly defect-free InGaN/GaN dot-in-a-wire heterostructures on silicon could be implemented. Further, by varying the growth parameters to adjust the In composition within the InGaN quantum dots these high efficiency optical emitters (approximately 45%) may be beneficially tuned to emit within the green, yellow, and amber/red regions of the visible spectrum to compliment the already existing blue and blue-green emission sources. Beneficially the nearly defect-free growth permits the formation of In-rich nanoclusters to form through phase separation within the InGaN quantum dot, such that these high efficiency optical emitters are further implemented by a unique dot-within-a-dot-in-a-wire structure rather than a prior art dot-in-a-wire approach.
Additionally, if multiple quantum dots are incorporated with high quality, uniform InGaN nanowire geometries vertically such that the InGaN nanowire device actually monolithically stacks multiple, for example blue, green and red, emitters in a single device which can then be realised with a single epitaxial growth sequence. Beneficially embodiments of the invention therefore allow for phosphor-free white LEDs that can be fabricated on low cost, large area substrates with high luminous flux.
Although progress has been made for InGaN/GaN quantum well LEDs, the performance of such devices in the green, yellow, and red wavelength ranges has been plagued by the very low efficiency, as outlined supra, and “efficiency droop”, i.e. the decrease of the external quantum efficiency with increasing current. The underlying mechanism has been extensively investigated and includes factor such as defects and carrier delocalization, polarization field, Auger recombination, carrier leakage, and poor hole transport. To this end, intensive studies have been performed with the use of various nanostructures, including quantum dots and nanowires, which can exhibit significantly reduced dislocation densities and polarization field and provide a greater degree of flexibility for sophisticated device engineering, compared to conventional planar heterostructures. Multi-color emission has been realized by using InGaN/GaN core-multi-shell and well/disk-in-a-wire structures and by exploring various hybrid nanowire heterostructures, see for example “Fabrication of a High-Brightness Blue-Light-Emitting Diode Using a ZnO-Nanowire Array Grown on p-GaN Thin Film” by X. M. Zhang et al (Adv. Mater. 2009, Vol. 21, p. 2767, 2009) and “GaN/InxGa1-xN//ZnO Nanoarchitecture Light Emitting Diode Microarrays” by C-H Lee et al (Appl. Phys. Lett., Vol. 94, p. 213101, 2009).
More recently, white light emission has been demonstrated in LEDs consisting of compositionally graded InGaN nanowires (see for example “Catalyst-Free InGaN/GaN Nanowire Light Emitting Diodes Grown on (001) Silicon by Molecular Beam Epitaxy” by W. Guo et al, Nano Lett., Vol. 10, p. 3355, 2010), InGaN/GaN nanodisks (see for example “InGaN/GaN Nanorod Array White Light-Emitting Diode” by H-W Lin et al, Appl. Phys. Lett. Vol. 97, p. 073101, 2010), and etched InGaN quantum wells (“High Performance InGaN/GaN Nanorod Light Emitting Diode Arrays Fabricated by Nanosphere Lithography and Chemical Mechanical Polishing Processes” by Y-L Chen, Opt. Express, Vol. 18, p. 7664, 2010).
However, a significant roadblock for the development of nanowire LEDs is the very low quantum efficiency, and the fact that to date, there has been no report on the internal quantum efficiency of GaN-based nanowire LEDs under electrical injection. Direct electrical injection being beneficial for high efficiency to avoid cascading efficiencies of photon generation and then optically pumping the quantum heterostructures. Due to the lack of 3-dimensional carrier confinement, the radiative electron-hole recombination in commonly reported GaN nanowire LED heterostructures has been severely limited by the presence of unoccupied Ga dangling bond and/or large densities of surface defects along the nonpolar GaN surface (m-plane), which can lead to a Fermi-level pinning on the nanowire lateral surfaces. Additionally, the device performance is adversely affected by the poor hole injection and transport processes in InGaN/GaN nanoscale heterostructures, caused by the heavy effective mass, small mobility, and low concentration of holes. While electrons can exhibit a relatively uniform distribution across the entire active region, injected holes largely reside in the small region close to the p-doped GaN layer. The highly non-uniform carrier distribution also lead to significantly enhanced Auger recombination and increased electron overflow, further limiting the optical emission efficiency at high injection levels
In this regard, special techniques, including p-doped active region, electron blocking layer, and thin InGaN barriers, have been implemented with the prior art to improve the performance of conventional InGaN/GaN quantum well LEDs. However, such phenomena have not been addressed for nanowire LEDs. Accordingly, in this context, it would be beneficial for there to be a method of growing and fabricating dot-in-a-wire LED heterostructures grown on Si(111) substrates that remove the efficiency bottleneck of nanowire devices. It would be further advantageous if through varying the epitaxial growth process that the superior carrier confinement provided by the dots could be combined with a significantly enhanced hole transport, and that this be achieved in a single epitaxial growth sequence.
Whilst the discussions above have centered to the benefits of high quality uniform nanowires grown without foreign metal catalysts have centered to broadband optical emitters, detectors, and solar cells these benefits of monolithically integrated high quality optical emitters, electrodes, PIN diodes, heterostructures, etc also allow such structures to be exploited in photoelectrochemical processes as well as photocatalysis, for example the generation of hydrogen and oxygen from water through solar energy, as well as other areas including for example electrobiological devices for sensing.
Accordingly it would be beneficial to provide method of combining multiple quantum dots with the high quality, uniform InGaN nanowires that are grown directly upon silicon substrates without the requirement for predeposition of a catalyst. Such a combination beneficially provides for InGaN/GaN dot-within-a-dot based nanowire photonic devices including photodetectors, solar cells, and optical emitters allowing device with high efficiency to be implemented across the entire visible spectrum.
According to embodiments of the invention therefore high efficiency photonic devices for generating high efficiency solid state white light sources or high efficiency solid state broad spectrum photodetectors can be manufactured based upon forming nanowires using a nearly-defect free InN, GaN, or AlN process upon silicon and other substrates that does not require a foreign metal catalyst to be introduced or complex precursor gas control processes to achieve growth which would be incompatible with forming either quantum dot in a wire structures or the quantum dot within a quantum dot within a wire structures for increased efficiency.
Hydrogen production is a large and growing industry where today primary demands from the refinery and ammonia industries have out global production at approximately 53 million metric tons in 2010. The global hydrogen generation market is anticipated to grow from approximately $88 billion in 2011 to approximately $118 billion by 2016 (see ADR Reports “Hydrogen Generation Market—by Merchant & Captive Type, Distributed & Centralized Generation, Application & Technology—Trends & Global Forecasts (2011-2016)”, December 2011). Within the ammonia industry the hydrogen is used in the Haber process to produce ammonia (NH3), which is then used directly or indirectly as fertilizer. Within the refinery industry the hydrogen is used to convert heavy petroleum sources into lighter fractions suitable for use as fuels through hydrocracking. However, hydrogen also forms the basis of the so-called “hydrogen economy” which is proposed as a solution to the problems that the current fossil fuel economy generates as well as addressing the post-fossil fuel era where economically viable fossil fuel reserves have been exhausted.
Within the hydrogen economy the exploitation of hydrogen based fuel cells provides for the generation of electricity in either stationary or portable applications via the oxidation of hydrogen. Accordingly, the only byproduct is water. Further, where the hydrogen is generated by electrolysis then the generation of the hydrogen adds no greenhouse gases to the environment. There is a so-called “perfect cycle” in that electrolysis produces hydrogen from water, and the hydrogen recombines with oxygen to create water and electrical power in a fuel cell. As such hydrogen based fuel cells in conjunction with electrolytic hydrogen generation provide for a hydrocarbon free electrical generation process and further when used in conjunction with electrical motors a hydrocarbon free mechanical motion generation process. Further, as hydrogen can be produced anywhere that you have electricity and water then electricity generation can be distributed reducing infrastructure demands and losses in electrical transmission.
Despite the attractiveness of this approach one of the key obstacles is the availability of hydrogen. Currently, global hydrogen production is 48% from natural gas, 30% from oil, and 18% from coal; water electrolysis accounts for only 4%. Clearly using fossil fuel sources negates any environmental benefits of hydrogen as an energy source and even if the hydrogen economy never materializes, the production of hydrogen for industrial consumptions still consumes large amounts of energy. Accordingly, it would be highly beneficial to produce hydrogen using a clean, renewable energy source and hence exploiting broad spectrum high efficiency nanowire based photodetectors/solar cells has significant benefit to hydrogen generation economics and logistics.
Splitting water directly from solar irradiation further allows this to be achieved without the consumption of electric power and accordingly since the initial reporting of the Honda-Fujishima effect using a TiO2/Pt photoelectrochemical cell in the early 1970's the use of semiconductors for photocatalytic water splitting has attracted tremendous interest, see Fujishima et al in “Electrochemical photolysis of water at a semiconductor electrode” (Nature, Vol. 238, pp. 37-38). Semiconductor photocatalytic water splitting generally involves three fundamental processes, these being the band gap absorption of photons and excitation of electron-hole pairs, the separation and migration of these photo-generated charge carriers, and the used of the photo-generated electrons and holes within surface redox reactions
Thermodynamically, if the conduction band minimum is more negative than the reduction potential of H+/H2 (0V versus normal hydrogen electrode (NHE)) and the valence band maximum is more positive than the oxidation potential of O2/H2O (1.23V vs. NHE), then water molecules can be reduced by electrons to form H2 and oxidized by holes to form O2 to achieve overall water splitting. Accordingly, in the intervening 40 years the development of photocatalysis for water splitting has primarily focused upon large bandgap metal oxides involving ions with filled or empty d-shell bonding configurations, e.g. Ti4+, Zr4+, Nb5+, Ta5+, W6+, Ga3+, I3+, Ge4+, Sn4+ and Sb5+, and oxynitrides such as (Ga1-xZnx)(N1-yOy).
Since the work by Fujishima photocatalytic water splitting has attracted considerable attention, see for example Tachibana et al in “Artificial photosynthesis for solar water-splitting” (Nature Photon., Vol. 6, pp 511-518) and Maeda et al in “Photocatalytic Water Splitting: Recent Progress and Future Challenges” (J. Phys. Chem. Lett., Vol. 1, Iss. 18, pp 2655-2661), as it allows direct conversion of solar energy into hydrogen. During the intervening 40 years research has largely focused upon metal-oxide based photocatalyst materials, see for example Kudo et al in “Heterogeneous Photocatalyst Materials for Water Splitting” (Chem. Soc. Rev., Vol. 38, pp. 253-278) and Guo et al in “Solar Hydrogen Generation: Transition Metal Oxides in Water Photoelectrolysis” (McGraw Hill, 2012) which are responsive to ultraviolet (UV) light, which accounts for approximately 4% of the solar spectrum at Earth's surface. Success in finding an abundant visible light active material, however, has been very limited, see for example Maeda in “Photocatalytic Water Splitting using Semiconductor Particles: History and Recent Developments” (J. Photochem. Photobiol. C: Photochem. Rev., Vol. 12, Iss. 4, pp. 237-268). Visible light being preferable for solar based water-splitting as it accounts for approximately 43% of the solar spectrum at the Earth's surface.
This is attributed to the lack of a known single photocatalyst material, one-step system, which has sufficiently narrow bandgap (<3 eV) to harness visible photons, suitable band-edge potentials for overall water splitting (i.e. simultaneous production of H2 and O2), and has a high level of stability against photocorrosion. In this regard, various energy band engineering methods have been explored to transform UV-active materials into visible-light active photocatalyst, see for example Tong et al. in “Nano-Photocatalytic Materials: Possibilities and Challenges” (Adv. Mater., Vol. 24, No. 2, pp. 229-251), including modulation techniques of the valence band. Such techniques including doping with 3d-transition metal cations, see for example Zou et al in “Direct Splitting of Water under Visible Light Irradiation with an Oxide Semiconductor Photocatalyst” (Nature, Vol. 414, pp. 625-627, hereinafter Zoul), cations with d10 or d10 s2 configurations, see Kim et al in “An Undoped, Single-Phase Oxide Photocatalyst Working under Visible Light” (J. Am. Chem. Soc., Vol. 126, pp. 8912-8913), and various anions, see for example Khan et al in “Efficient Photochemical Water Splitting by a Chemically Modified n-TiO2” (Science, Vol. 297, pp. 2243-2245), Asahi et al in “Visible-Light Photocatalysts in Nitrogen-doped Titanium Oxides” (Science, Vol. 293, pp. 269-271), and Chen et al in “The Electronic Origin of Visible Light Absorption Properties of C-, N-, and S-doped TiO2 Nanomaterials” (J. Am. Che. Soc., Vol. 130, pp. 5018-5019).
Other approaches have included adjustment of conduction band, e.g. incorporation of p-block cations with d10 configuration, see for example Zou et al in “Substitution Effects of In3+ by Al3+ and Ga3+ on the Photocatalytic and Structural Properties of the Bi2InNbO7 Photocatalyst” (Chem. Mater., Vol. 13, pp. 1765-1769), and continuous modulation of valence and/or conduction band, e.g. by solid solution, see for example Maeda et al. in “GaN:ZnO Solid Solution as a Photocatalyst for Visible-Light-Driven Overall Water Splitting” (J. Am. Chem. Soc., Vol. 127, pp. 8286-8287), and disorder engineering, see for example Chen et al in “Increasing Solar Absorption for Photocatalysis with Black Hydrogenated Titanium Dioxide Nanocrystals” (Science, Vol. 331, pp. 746-750). However, most of these approaches cannot drive stable overall pure water splitting beyond the blue wavelength range, see for example Zoul and Maeda et al. in “Photocatalyst Releasing Hydrogen from Water” (Nature, Vol. 440, pp. 295).
However, more recently, the use of group-III nitride semiconductors for water splitting has attracted considerable attention due to the more negative potential of the nitrogen 2p-orbital compared to that of oxygen 2p-orbital and that metal-nitrides often possess a narrow bandgap resulting in narrowband absorption. In contrast group-III nitride semiconductors could potentially encompass nearly the entire solar spectrum at the Earth's surface. Moreover, the inherent chemical stability of nitrides also favors the harsh photocatalysis reaction environment. Indeed, recent first-principles calculations suggest that a single H2O molecule can be efficiently cleaved in an exothermic reaction to form H2 under photo-excitation at Ga-terminated surface sites. Ab initio molecular dynamic simulations further show that the overall water oxidation reaction at GaN surfaces can be energetically driven by photogenerated holes.
Within the prior art it is known that the size, morphology, surface chemistry, and crystal structure of photocatalysts often play a crucial role in determining their photophysical and photocatalytic properties. Further within the prior art photocatalysts are typically employed in the form of powders to provide large surface area within a small volume. However, photocatalysts in the form of one-dimensional (1-D) nanostructures, such as nanowires, nanobelts, and nanotubes, are highly desired as these exhibit extremely large surface-to-volume ratios and significantly enhanced light absorption. More importantly, in 1-D nanostructured single crystals charge carriers are delocalized and therefore free to transport throughout the whole length dimension. Consequently, significantly enhanced photocatalytic activity is expected from 1-D nanoscale materials.
To date, such semiconductor nanowire based devices, such as those exploiting InN, GaN, and AlN, have generally been achieved on single crystalline substrates. However, it would be beneficial in many applications to reduce the device fabrication cost and achieve seamless integration with other device components. Accordingly, with the current availability of lower cost large area amorphous substrates and the emergence of high perfoiinance flexible photonics and electronics it would be beneficial for high-quality semiconductor nanowires and nanowire based devices to act as the building blocks for future photonic and electronic devices on amorphous and/or flexible substrates. Further, flexibility in substrate allows wider design aspects, e.g. thermal management, chemical resistance, and high speed electronics for example, as well as tailoring the substrate to the device design and application for example for enhanced performance.
According to embodiments of the invention therefore high efficiency LEDs for generating high efficiency solid state white light sources can be manufactured based upon forming nanowires, in conjunction with the carrier confinement enabled by the dot-in-a-wire structures and the enhanced carrier transport by the modulation p-doping technique, using a nearly-defect free InGaN nanowire growth process upon silicon substrates that does not require a foreign metal catalyst to be introduced or complex precursor gas control processes to achieve growth which would be incompatible with forming either quantum dot in a wire structures or the quantum dot within a quantum dot within a wire structures for increased efficiency. The process further allow single epitaxial growth and enhanced hole transport through controlled doping of the structures.